The development of suitable lithography processes for producing very fine structures, for example of a size of less than 100 nm, is presenting extraordinarily difficult problems. Some of these problems result from the resist chemistry, the mask production and the complexity of the lithography system.
Optical lithography for producing very fine structures in a range of less than 100 nm has reached 157 nm lithography processes. These lithography processes require new types of resist materials. However, despite the most intensive efforts, a resist which completely satisfies the technical demands with regard to such small structures has not yet been found. Furthermore, in addition to these new materials, new processes for mask production are also required. The development of these processes is once again very cost-intensive. Therefore, very cost-intensive lithography systems that are difficult to handle are the result.
Consequently, sublithographic processes have been introduced as an alternative to conventional optical lithography processes of the above type. In these sublithographic processes, a structure is formed photolithographically on an auxiliary layer, for example using a conventional photoresist, the auxiliary layer is anisotropically etched using the patterned photoresist, the resist mask is removed, and then the auxiliary layer is etched back again from all sides by means of an isotropic etching process and is thereby reduced in size. This gives sublithographic mask structures which can be transferred to a gate layer using standard etching processes to form, for example, sublithographic gate structures.
In the same way, sublithographic mask structures of this type can also be formed by means of a spacer process. In a spacer process, a first mask with substantially vertical side walls is formed and patterned, usually by means of optical lithography. Then, a very thin second mask layer is deposited over the entire surface of the first mask up to a predetermined thickness. Next, the horizontal layer regions of the second mask layer are removed by means of an anisotropic etching process, so that only a sublithographic mask structure remains on the side wall of the first mask. Finally, the first mask is removed and the isolated sublithographic mask structures with their predetermined thickness or gate length are transferred into the gate layer below to form, for example, a sublithographic gate structure.
However, a drawback of conventional processes of this type is that although it is in this way possible to produce sublithographic structures, the minimum distance between these structures continues to be determined by the lithographic system. The minimum distance is greater than the minimum feature size F that can be produced by means of lithography. More specifically, conventional processes of this nature continue to have a pitch, which is composed of the sum of a structure width and a distance between directly adjacent structures, which is still 2 F, i.e. double the minimum feature size that can be produced by means of lithography.
Therefore, to provide sublithographic structures in which a distance between adjacent structures also has sublithographic dimensions, as well as a process for producing the sublithographic structures.
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